TRANSPORTER OF IBA1 links auxin and cytokinin to influence root architecture ($) (Devel. Cell)

Indole-3-butyric acid (IBA) is the precursor of the hormone auxin and it controls the formation of lateral roots. Evidence suggests that IBA is converted to IAA, endogenous active auxin. The major study material to distinguish between IBA and IAA was highlighted by the IBA-specific efflux carrier ABCG36/PDR8/PEN3, and abcg36 mutant hyperaccumulates IBA in the root. Michniewicz et al. performed a suppressor screening using pen3-4, an allele of ABCG36, to identify additional IBA transporters, and from their screening, they found TRANSPORTER OF IBA1 (TOB1) as a tonoplast-localized IBA transporter. tob1 mutants are resistant to both IBA and its synthetic analog 2,4-dichlorophenoxy butyric acid (2,4-DB). Interestingly, along with IBA and 2,4-DB, it also transports nitrate and is also known as NPF5.12. Additionally, tob1 mutants have higher lateral root density, consistent with defects in IBA transport Furthermore, the authors showed that TOB1 expression is regulated by cytokinin. Altogether, this work illustrates a nice working model focusing on TOB1, which integrates auxin homeostasis and cytokinin response to maintain root system architecture. (Summary by Arif Ashraf) Developmental Cell 10.1016/j.devcel.2019.06.010